1.Field of the Invention
The present invention relates to X-ray apparatuses, and more particularly, within such apparatuses, it relates to a device or filter for spatially modifying exposure to X-rays as a function of the morphology of the patient's body or of the portion of the body receiving X-rays.
2.Description of the Prior Art
The invention is more particularly intended for X-ray apparatuses that are used for angiographical examination of the lower limbs. It is recalled that angiography is the application and adaptation of the radiological technique to the vascular network: arteries, veins, tissues under perfusion. It makes use of "contrast" liquids based on iodine which are opaque to X-rays and which are injected into the vascular network in order to enable it to be visualized by distinguishing it from the surrounding tissue. More precisely, the patient is laid on a table which is designed to move relative to a source of radiation associated with a receiver disposed on the other side of the patient from the source. The practitioner injects the X-ray opaque liquid, known as the "contrast" liquid, into an artery or a vein of the patient lying on the table. Then, a few seconds after the injection, a plurality of successive X-ray pictures of the patient are taken so as to visualize and measure the progress of the contrast substance along the blood vessels as a function of time.
When angiographic examination is applied to the lower limbs, i.e. over a length of about 120 centimeters, several methods are currently in use.
The first method consists in using an "arteriophlebograph" as the receiver equipment, which equipment comprises a hexagonal drum with six film and reinforcing screen pairs provided thereon, each pair being 120 cm long and 35 cm wide, thereby enabling six exposures and thus six photographic pictures to be obtained at different instants. In this method, each picture provides an image over the full length of the limbs and the patient therefore receives a considerable dose of X-rays since the entire lower portion of the body is exposed when each picture is taken. In addition, since the exposure is the same over the entire lower portion of the body, there is a large difference in contrast between the abdomen and the feet, given the differences in X-ray absorption presented by these portions of the body. This large difference in contrast makes it difficult to identify the contrast substance. In order to reduce this contrast difference, proposals have been made to place a rudimentary filter on the path of the X-ray beams, the filter being of the wedge type, i.e. being constituted by a sheet of varying thickness, thereby attenuating X-rays at the feet more than at the abdomen. Proposals have also been made to use a reinforcing screen whose X-ray to light photon efficiency varies over the length of the lower limbs, so as to present low efficiency at the feet and medium to high efficiency at the abdomen. The changes in contrast that are obtained using such devices, i.e. a wedge-shaped plate or a varying efficiency reinforcing screen, are fairly coarse.
A second method consists in examining the lower limbs zone by zone, with each zone corresponding, for example, to an effective field of 35 cm.times.35 cm or to a diameter of 30 cm or of 35 cm. To do this, relative displacement is obtained between the X-ray beam and the patient between taking successive pictures such that each picture gives a partial image of the limbs, and the resulting set of images covers the entire length of the limbs.
In this second method, the exposure parameters vary from one picture to the next in order to take into account of the variations in thickness between the abdomen and the feet. In practice, given the dynamic range of the variations required, it is necessary to vary both the supply voltage kV applied to the tube of the X-ray source and the product mA.s of the anode current mA multiplied by the exposure time s. This therefore results in a change in the contrast between the images taken of different zones of the patient, and this makes identifying the contrast substance difficult, and more generally makes analyzing the images difficult, particularly if they are processed digitally.
In both methods outlined briefly above, some portions of the sensitive surface of the receiver, corresponding to the gap between the legs or to margins outside the legs are subjected to X-rays which are unattenuated and this degrades image quality. This problem is solved by placing various absorbent bodies such as plastic cylinders filled with water around the patient and between the patient's legs. Manipulating such cylinders is inconvenient both for the patient and for the practitioner.
The object of the present invention is therefore to provide an X-ray apparatus which avoids the above-mentioned drawbacks. This result is achieved by using a filtering or absorption device which is disposed between the source of radiation and the receiver, and preferably in the vicinity of the source.